بررسي رفتار خوردگي بين دانه‌اي فولاد زنگ نزن مارتنزيتي UNS S42000 با استفاده از طيف‌سنجي امپدانس الكتروشيميايي پتانسيوديناميك

Investigation of the Intergranular Corrosion Behavior of UNS S42000 Martensitic Stainless Steels Using Potentiodynamic Electrochemical Impedance Spectroscopy

در اﯾﻦ ﭘﮋوﻫﺶ، ﻋﻤﻠﯿﺎت ﺣﺮارﺗﯽ ﺗﻤﭙﺮ در دﻣﺎي ﺛﺎﺑﺖ 550 درﺟﻪ ﺳﺎﻧﺘﯽ ﮔﺮاد و زﻣﺎن ﻫﺎي ﻣﺘﻔﺎوﺗﯽ از 0/5 ﺗﺎ 24 ﺳﺎﻋﺖ ﺑﺮ روي ﻧﻤﻮﻧﻪ ﻫﺎي ﻓﻮﻻد زﻧﮓ ﻧﺰن ﻣﺎرﺗﻨﺰﯾﺘﯽ UNS S42000 ﺻﻮرت ﮔﺮﻓﺖ ﺗﺎ ﺑﺘﻮان رﻓﺘﺎر ﺧﻮردﮔﯽ ﺑﯿﻦ داﻧﻪ اي آﻟﯿﺎژ را ﻣﻮرد ﺑﺮرﺳﯽ ﻗﺮار داد. ﺧﺼﻮﺻﯿﺎت ﻣﯿﮑﺮوﺳﺎﺧﺘﺎر و ﺧﻮاص اﻟﮑﺘﺮوﺷﯿﻤﯿﺎﯾﯽ آﻟﯿﺎژ ﺑﻪ ﺗﺮﺗﯿﺐ ﺑﺎ اﺳﺘﻔﺎده از ﻣﯿﮑﺮوﺳﮑﻮپ اﻟﮑﺘﺮوﻧﯽ روﺑﺸﯽ و روش ﭘﻼرﯾﺰاﺳﯿﻮن آﻧﺪي ارزﯾﺎﺑﯽ ﺷﺪﻧﺪ. ﺧﻮردﮔﯽ ﻣﻮﺿﻌﯽ ﻧﻤﻮﻧﻪ ﻫﺎ ﻧﯿﺰ ﺑﺎ اﺳﺘﻔﺎده از روش ﻃﯿﻒ ﺳﻨﺠﯽ اﻣﭙﺪاﻧﺲ اﻟﮑﺘﺮوﺷﯿﻤﯿﺎﯾﯽ ﭘﺘﺎﻧﺴﯿﻮدﯾﻨﺎﻣﯿﮏ در ﻧﺎﺣﯿﻪ ﭘﺘﺎﻧﺴﯿﻞ ﺗﺮاﻧﺲ ﭘﺴﯿﻮ ﺻﻮرت ﮔﺮﻓﺖ. ﻧﺘﺎﯾﺞ ﺣﺎﮐﯽ از ﺧﻮردﮔﯽ ﺑﯿﻦ داﻧﻪ اي ﺷﺪﯾﺪي ﺑﺮاي ﻧﻤﻮﻧﻪ ﻫﺎﯾﯽ ﺑﺎ 2 ﺳﺎﻋﺖ ﻋﻤﻠﯿﺎت ﺣﺮارﺗﯽ ﺗﻤﭙﺮ ﺑﻮد، درﺻﻮرﺗﯽ ﮐﻪ زﻣﺎن ﻫﺎي ﻃﻮﻻﻧﯽ ﻋﻤﻠﯿﺎت ﺣﺮارﺗﯽ ﺗﻤﭙﺮ ﺑﻪ دﻟﯿﻞ ﺣﺴﺎﺳﯿﺖ زداﯾﯽ ﻣﻮﺟﺐ ﺗﻘﻠﯿﻞ اﺛﺮ ﺗﻤﭙﺮ ﺑﺮ ﺧﻮردﮔﯽ ﺑﯿﻦ داﻧﻪ اي ﻣﯽ ﺷﻮد. ﻋﻼوه ﺑﺮ اﯾﻦ، ﻧﺘﺎﯾﺞ ﻃﯿﻒ ﺳﻨﺠﯽ اﻣﭙﺪاﻧﺲ اﻟﮑﺘﺮوﺷﯿﻤﯿﺎﯾﯽ ﭘﺘﺎﻧﺴﯿﻮدﯾﻨﺎﻣﯿﮏ ﻧﺸﺎن داد ﮐﻪ ﻣﻘﺎوﻣﺖ اﻧﺘﻘﺎل ﺑﺎر ﻧﻤﻮﻧﻪ ﻫﺎ در ﭘﺘﺎﻧﺴﯿﻞ 1/10 وﻟﺖ ﻣﯽ ﺗﻮاﻧﺪ ﺑﻪ ﻋﻨﻮان ﻣﻌﯿﺎر ﻣﻬﻤﯽ ﺑﻪ ﻣﻨﻈﻮر ﺑﺮرﺳﯽ ﺧﻮردﮔﯽ ﺑﯿﻦ داﻧﻪ اي ﻓﻮﻻدﻫﺎي زﻧﮓ ﻧﺰن ﻣﺎرﺗﻨﺰﯾﺘﯽ ﻣﻮرد اﺳﺘﻔﺎده ﻗﺮار ﮔﯿﺮد، ﺑﺪﯾﻦ ﺻﻮرت ﮐﻪ ﻫﺮﭼﻪ ﻣﻘﺎوﻣﺖ اﻧﺘﻘﺎل ﺑﺎر در اﯾﻦ ﭘﺘﺎﻧﺴﯿﻞ ﮐﻤﺘﺮ ﺑﺎﺷﺪ، ﻣﻘﺎوﻣﺖ ﺑﻪ ﺧﻮردﮔﯽ ﺑﯿﻦ داﻧﻪ اي ﻧﯿﺰ ﮐﻤﺘﺮ ﺧﻮاﻫﺪ ﺑﻮد. اﯾﻦ ﻧﺘﺎﯾﺞ در ﺗﻄﺎﺑﻖ ﺑﺎ ﺗﺼﺎوﯾﺮ ﻣﺮﺑﻮط ﺑﻪ ﻣﻮرﻓﻮﻟﻮژي ﺳﻄﻮح ﺧﻮرده ﺷﺪه ﺑﻮده ﮐﻪ ﺑﯿﺎﻧﮕﺮ ﺧﻮردﮔﯽ ﯾﮑﻨﻮاﺧﺖ، ﺧﻮردﮔﯽ ﻣﻮﺿﻌﯽ و ﺣﺴﺎﺳﯿﺖ زداﯾﯽ ﺑﻪ ﺗﺮﺗﯿﺐ ﺑﺮاي ﻧﻤﻮﻧﻪ ﻫﺎي ﺗﻤﭙﺮ ﻧﺸﺪه، زﻣﺎن ﻫﺎي ﮐﻮﺗﺎه ﺗﻤﭙﺮ و زﻣﺎن ﻫﺎي ﻃﻮﻻﻧﯽ ﺗﻤﭙﺮ ﻣﯽ ﺑﺎﺷﺪ.

In this research, UNS S42000 martensitic stainless steel (SS) samples were subjected to tempering heat treatment at constant temperature of 550 oC for different time duration, ranging between 0.5 and 24 h, so as to assess the intergranular corrosion (IGC) behavior of the alloy. The microstructural characteristics and electrochemical properties of the alloys were evaluated using scanning electron microscopy (SEM) and anodic polarization method, respectively. The localized attack of the samples was investigated by potentiodynamic electrochemical impedance spectroscopy (PD-EIS) in the transpassive potential region. The results indicated that tempering up to 2 h would be followed by severe intergranular attack at martensite lath interfaces; however, the effect of tempering on corrosion performance of the alloys would be suppressed at prolonged tempering due to desensitization. Moreover, the PD-EIS results revealed that the charge transfer resistance (Rct) values of specimens at 1.10 V can be employed as a primary criterion in order to assess the localized attack of the martensitic SS. In this regard, the lower the Rct value at this DC bias potential, the lower the corrosion resistance to the IGC. These results were in good agreement with the SEM morphologies from the surfaces of corroded samples, indicating uniform corrosion, severe attack and desensitization for samples experiencing no-tempering, short tempering time and prolonged tempering, respectively.